WO2012163530A1

WO2012163530A1 - Components of plant, such as reduction furnace body and/or electrode, in particular for a reduction furnace

Info

Publication number: WO2012163530A1
Application number: PCT/EP2012/002307
Authority: WO
Inventors: Lars Nygaard; Birger Andresen; Arvid Inge SØRVIK
Original assignee: Evonik Solar Norge AS
Current assignee: Evonik Solar Norge AS
Priority date: 2011-06-03
Filing date: 2012-05-31
Publication date: 2012-12-06
Anticipated expiration: 2013-12-03
Also published as: EP2530051A1

Abstract

Component of a plant for the production of silicon, in particular liquid silicon and solar grade silicon, wherein the component is a reduction furnace body and/or an electrode that comprise a composition selected from (i) 80-95 % (w/w) of a silicon carbide product containing 1.5 ppm (w/w) or less boron and 3.0 ppm (w/w) or less phosphorus; (ii) 5-20 % (w/w) of a silicon product containing 1.5 ppm (w/w) or less boron and 3.0 ppm (w/w) or less phosphorus; and (iii) 0.1-5 % (w/w) of a binding agent, such as a resin, e.g. a synthetic resin; and, wherein the sum of this composition is 100 % (w/w) before a curing process, a process for the production as well as the use of the silicon containing material.

Description

COMPONENTS OF PLANT, SUCH AS REDUCTION FURNACE BODY AND/OR ELECTRODE, IN PARTICULAR FOR A REDUCTION FURNACE

Technical field

The present disclosure relates in particular to the field of production of solar grade silicon as well as to the processing of metals in an arc furnace. In particular, it relates to the electrodes of a reduction furnace, more preferred to electrodes of a reduction furnace for carrying out a carbothermic reduction of quartz. Object of the invention is therefore, a component of a plant for the production of silicon by carbothermic reduction, wherein the component is an inner lining of an reduction furnace or an electrode of a composition selected from (i) 80-95 % (w/w) of a silicon carbide product containing 1.5 ppm (w/w) or less boron and 3.0 ppm (w/w) or less phosphorus; (ii) 5-20 % (w/w) of a silicon product containing 1.5 ppm (w/w) or less boron and 3.0 ppm (w/w) or less phosphorus; and (iii) 0.1-5 % (w/w) of a binding agent, such as a resin, e.g. a synthetic resin; and, wherein the sum of this composition is 100 % (w/w) before a curing process, a process for the production as well as the use of the silicon containing material. Background

There is a growing demand for solar cell panels, partly due to the concerns about global warming. Accordingly, there is also a growing demand for a cost-efficient and environmentally friendly (i.e. energy-efficient) method for preparing the silicon needed to produce the solar cell panels. There are several ways of producing silicon. Among these are; A chemical method involving acid treatment of metallurgical silicon followed by a gas phase reaction with HCI and gas phase decomposition on silicon rods with hydrogen. This chemical method, known as the Siemens Process, consumes high amounts of energy and is thus expensive. A physical purification method involving refining semi-pure liquid silicon with slag and gas followed by solidification is performed. The solid product is crushed and Fe, Al, Ca and Ti are leached out. The product is then re-melted and subjected to directional solidification. This method is used by the Elkem Company. In a third method, pure quartz is subjected to carbothermic reduction to obtain a silicon liquid, which may be treated by the following steps; gas refining, temperature adjustment, settling, filtering, skimming and directional solidification. A challenge using this method is to produce a silicon product of sufficient purity.

Diclosure of Invention

The inventors have realized that a method based on a carbothermic reduction of quartz, followed by refining in the liquid state is a promising alternative for the production of solar grade silicon. But general problems for the carbothermic reduction to silicon are the accumulation of impurities and energy consumption. In particular due to the high processing temperature during the reduction in an arc furnace, metallic impurities will be released from the linings of the furnaces because their mobility is increased at high temperatures and therefore impurities tend to be released into liquid silicon from furnace parts in contact with said silicon.

In comparison to the state of the art processes, the carbothermic reduction process is associated with low investment costs, low operational costs and low energy consumption compared to the physical purification method described above, fewer impurities are removed after the carbothermic reduction in this method, and instead, the inventors have chosen to use starting materials of higher purity. The inventors have concluded that it is therefore important that the liquid silicon is not contaminated during the carbothermic reduction. In particular, the inventors have found that if the inner lining of the reduction furnace in which the carbothermic reduction is carried out has a certain composition of components of certain purities, the product of the carbothermic reduction is not contaminated. In addition, the inventors have found that if crucial parts or inner linings of components of a plant for the production of solar grade silicon are furnished with high purity according to boron, phosphorus and other metals a contamination with other metals than silicon can predominately avoided. Therefore the inventors have found a useful certain composition which led to stable components under these high temperatures, aggressive and eroding conditions.

In addition, the inventors have found that if the electrode and/or at least some components of an electrode, such as electrode body, power connection, nipple, blocking pin, mounting, clamping jaws with which the reactants or the movable electrode are in contact during the carbothermic reduction have a certain composition of components of certain purities, the product of the carbothermic reduction is not contaminated. Thus, a silicon product having the characteristics demanded by the solar cell producers may be obtained after a few relatively uncomplicated and energy-efficient refining steps in the liquid state following the carbothermic reduction.

Also it is particularly difficult and thus expensive to remove boron and phosphorus refining steps after the carbothermic reduction step. Thus, it is particularly important that the product of the carbothermic reduction is not contaminated by such elements.

The object of the invention was therefore, to furnish further components of a plant for production of solar grade silicon, in particular components of an arc furnace with a very low contamination with impurities such as boron, phosphorus and in particular metallic impurities, preferred all metals excluded silicon. In addition these parts should be heat resistant, and possess certain conductivity at high temperatures. A further object was to provide components which do less erode during its use or do not reduce the yield. A third object was to furnish a process for the production of these components. Further the components shall have a longer shelf life. Therefore embodiment of the invention is a component of a plant for production of silicon, in particular of liquid silicon and solar grade silicon solar grade silicon by carbothermic reduction, wherein said component comprises:

(i) 80-95 % (w/w) of a silicon carbide product containing 1.5 ppm (w/w) or less boron, and 3.0 ppm (w/w) or less phosphorus;

(ii) 5-20 % (w/w) of a silicon product containing 1.5 ppm (w/w) or less boron and 3.0 ppm (w/w) or less phosphorus; and

(iii) 0.1-5 % (w/w) of a reaction product of the binding agent and/or binding agent, such as a resin, e.g. a synthetic resin; and, wherein the sum of this composition is 100 % (w/w).

Preferred components of a plant for production of liquid silicon by carbothermic reduction are selected from the inner lining of a reduction furnace body, inner lining of SiC furnace, inner lining of heating or drying oven, inner lining of pelletizing, tabletting, briquetting equipment, inner lining of extruder, electrode component, electrode, tapping channel, supply channel, connection parts, component of tapping equipment, ladle, crucible, inner lining of holding furnace, inner lining of directional solidification furnace, lining of devices that are in contact with quartz, silicon carbide, silicon products or formulation comprising any of quartz, silicon carbide or silicon product, inner lining of supply channels and inner lining of supply pipes.

Particular independently (i) and/or (ii) containing 1.0 ppm (w/w) or less boron, in particular equal or less than 0.7 ppm (w/w), preferred equal to less than 0.2 ppm (w/w) boron to 0.001 ppt (w/w), and 3.0 ppm (w/w) or less phosphorus, in particular equal or less than 0.5 ppm (w/w), preferred equal to less than 0.1 ppm (w/w) phosphorus to 0.001 ppt (w/w).

In particular component comprise as sum of all other metals (excluded silicon) in the silicon carbide product or the silicon product, respectively, is less than 100 ppm (w/w), wherein particularly said other metals are selected from Ag, Al, As, Au, Ba, Be, Bi, Ca, Cd, Ce, Co, Cr, Cs, Cu, Dy, Er, Eu, Fe, Ga, Gd, Ge, Hf, Hg, Ho, In, Ir, K, La, Li, Lu, Mg, Mn, Mo, Na, Nb, No, Os, Pb, Pd, Pr, Pt, Rb, Re, Rh, Ru, Sb, Sc, Se, Sm, Sn, Ta, Tb, Te, Th, Ti, Tl, Tm, U, W, Y, Yb, Zn, Zr or wherein the sum of all of said other metals in silicon carbide product and the silicon product is less 100 ppm (w/w), particularly less than 80 ppm (w/w), preferred less than 50 ppm (w/w), and in particular less than 10 ppm (w/w) iron.

Table 1 : A composition of a component according to the invention comprises:

Table 2: A composition of a component according to the invention comprises:

"content of metal ic elements in ppm (w/w)

Using high purity silicon for a composition of the invention with less than 5 ppm (w/w) in sum of metallic impurities, the overall content of metals in the paste may be below 80 ppm (w/w) or more preferred less than 50 ppm (w/w) to 0.0001 ppt (w/w).

More preferred is when in the component said other metals in the silicon carbide product and the silicon product are selected from the following is equal or less than: a. aluminium (Al) 30 ppm (w/w) or from 25 to 0.0001 ppt (w/w), particularly from 22 ppm (w/w) to 0.0001 ppt (w/w), and

b. boron (B) 1.0 ppm (w/w) to 0.0001 ppt (w/w) , particularly from 0.5 ppm (w/w) to 0.0001 ppt (w/w), preferred from 0.4 ppm (w/w) to 0.0001 ppt (w/w) or particularly preferred 0.35 ppm (w/w) to 10 ppb (w/w); and

c. calcium (Ca) 20 ppm (w/w), particularly from 15 to 0.0001 ppt (w/w) , preferred from 10 ppm (w/w) to 0.0001 ppt (w/w); and optional

d. gallium (Ga) 1 ppm, particularly from 0.5 ppm to 0.0001 ppt (w/w), preferred from 0.1 ppm (w/w) to 0.0001 ppt (w/w), particularly preferred 1 ppb (w/w) to 0.0001 ppt (w/w),

e. nickel (Ni) 10 ppm (w/w), particularly from 5 ppm (w/w) to 0.0001 ppt (w/w), preferred from 3.5 ppm und 0.0001 ppt (w/w);

f. phosphorus (P) 3 ppm (w/w) to 0.0001 ppt (w/w), particularly from 2 ppm (w/w) to 0.0001 ppt (w/w) .preferred from 1 ppm (w/w) to 0.0001 ppt (w/w), and

g. titan (Ti) 2 ppm (w/w), particularly from 1.7 ppm (w/w) to 0.0001 ppt, preferred from 1.5 to 0.0001 ppt (w/w); and h. zinc (Zn) 30 ppm (w/w), particularly from 25 ppm (w/w) to 0.0001 ppt (w/w) preferred from 10 ppm to 0.0001 ppt (w/w).

According to an embodiment the component is a green body and comprises a composition selected from (i) 80-95 % (w/w) of a silicon carbide product containing 1.5 ppm (w/w) or less boron and 3.0 ppm (w/w) or less phosphorus; (ii) 5-20 % (w/w) of a silicon product containing 1.5 ppm (w/w) or less boron and 3.0 ppm (w/w) or less phosphorus; wherein the silicon product is silicon, and (iii) 0.1-5 % (w/w) of a binding agent, such as a resin, a carbohydrate, e.g. a synthetic resin, a phenol- formaldehyde resins, and/or a saccharide; and, wherein the sum of this composition is 100 % (w/w). Typically the silicon has a purity of at least equal or greater than 99.9999 % (w/w), preferred 99.99999 % (w/w), more preferred equal or greater than 99.999999 % (w/w).

In particular the component, in particular cured and heated component/body, comprises a composition selected from (i) 80-95 % (w/w) of a silicon carbide product containing 1.5 ppm (w/w) or less boron and 3.0 ppm (w/w) or less phosphorus;

(ii) 5-20 % (w/w) of a silicon product containing 1.5 ppm (w/w) or less boron and 3.0 ppm (w/w) or less phosphorus; wherein the silicon product comprises silicon nitride and optionally silicon, and (iii) 0 to 5 (w/w) (w/w) of a reaction product of the binding agent, such as a resin, a carbohydrate, e.g. a synthetic resin and/or saccacharide; and, wherein the sum of this composition is 100 % (w/w).

According to an embodiment these parts be heat resistant, and possess certain conductivity at high temperatures. The electrical conductivity is measured to be 11 S/cm (T >1480 degree C), 2A (see table 3 electric conductivity (S/cm) of a tempered and/or heated SiC-composition at certain temperatures of samples (e.g. prove, 2A)). The heat conductivity is lower than 12 W/mK (T>900 degreeC), 2C 2E (see table 4, heat conductivity (W/mK)). Samples of table 5 are 2A, 2C, 2D, 2E and 2F. Table 4: electric conductivity (S/cm) of a tempered and/or heated SiC-composition at certain temperatures of samples

Table 5. heat conductivity

According to a second aspect of the invention one embodiment is a reduction furnace body for production of liquid silicon by carbothermic reduction, said body comprising an inner lining, characterized in that said inner lining comprises:

(i) 80-95 % (w/w) of a silicon carbide product containing 1.5 ppm (w/w) or less boron and 3.0 ppm (w/w) or less phosphorus;

(ii) 0.1-5 % (w/w) of a reaction product of the binding agent and/or binding agent, such as a resin, e.g. a synthetic resin; in particular the binding agent is a resin selected from synthetic phenol- formaldehyde resins;

and, in particular wherein the sum of this composition is 100 % (w/w). The reduction furnace body is particularly a component of a plant for production of silicon, liquid silicon and/or solar grade silicon by carbothermic reduction.

Also, there is provided a method for production of a silicon product containing 1.5 ppm (w/w) or less boron and 3.0 ppm (w/w) or less phosphorus by carbothermic reduction, comprising the steps of: Adding a quartz product containing 0.50 ppm (w/w) or less boron and 1.0 ppm (w/w) or less phosphorus and a silicon carbide product containing 1.5 ppm (w/w) or less boron and 3.0 ppm (w/w) or less phosphorus; to a reduction furnace having an inner lining; and b) heating the mixture such that a melt of the silicon product is formed, wherein said lining comprises: 80-95 % (w/w) of a silicon carbide product having at least the same purity as the silicon carbide product added in step a) 5-20 % (w/w) of a silicon product having at least the purity of the silicon product produced by the method; and 0.1-5 % (w/w) of a binding agent or a reaction product of the binding agent, such as a resin. In particular the sum of the disclosed compositions are 100 % (w/w).

Further, the a component of the plant, in particular the inner lining needs to stand the high temperatures of the carbothermic reduction, and the inventors have chosen the components of the component of the plant, in particular the inner lining or component of electrode, and their proportions, accordingly. Thus, 80-95 % (w/w), preferably 85-95 % (w/w), more preferably 87-93 % (w/w) of the component, particularly the electrode or inner lining is constituted by the carbide product. The carbide product contains 1.5 ppm (w/w) or less, preferably 1.0 ppm (w/w) or less, more preferably 0.5 ppm (w/w) or less, most preferably 0.2 ppm (w/w) or less B, preferred down below 0.00001 ppm (w/w). Further, the carbide product contains 3.0 ppm (w/w) or less, preferably 2.0 ppm (w/w) or less, more preferably 1.0 ppm (w/w) or less, most preferably 0.5 ppm (w/w) or less P, preferred down below 0.00001 ppm (w/w). Further, 5-20 % (w/w), preferably 5-15 % (w/w), more preferably 7-13 % (w/w) of the component, in particular the inner lining or electrode is constituted by the silicon product. The silicon product contains 1.5 ppm (w/w) or less, preferably 1.0 ppm (w/w) or less, more preferably 0.7 ppm (w/w) or less, most preferably 0.5 ppm (w/w) or less B. Further, the silicon product contains 3.0 ppm (w/w) or less, preferably 2.0 ppm (w/w) or less, more preferably 1.0 ppm (w/w) or less, most preferably 0.5 ppm (w/w) or less P.

According to one embodiment, the silicon carbide product contains 300 ppm (w/w), particularly less than 100 ppm (w/w) Al or less. According to an alternative or complementary embodiment, the silicon product contains 100 ppm (w/w) Al or less, such as 80 ppm (w/w) Al or less, preferred less than 70 ppm (w/w), less than 50 ppm (w/w).

As a first aspect of the present disclosure, there is thus provided a reduction furnace body for production of liquid silicon by carbothermic reduction, said furnace body comprising an inner lining, characterized in that said inner lining comprises 80-95 % (w/w) of a silicon carbide product containing 1.5 ppm (w/w) or less boron and 3.0 ppm (w/w) or less phosphorus; 5-20 % (w/w) of a silicon product containing 1.5 ppm (w/w) or less boron and 3.0 ppm (w/w) or less phosphorus; and 0.1-5 % (w/w) of a reaction product of the binding agent and/or a binding agent, such as a resin or a reaction product of the binding agent, e.g. a synthetic phenol formaldehyde resin.

In addition, wherein the reduction furnace body comprise a silicon carbide product or the silicon product each contains less than 300 ppm (w/w), less than 100 ppm (w/w), particularly less than 80 ppm (w/w) Al or less, particularly less than 50 ppm (w/w), preferred less than 30 ppm (w/w). A reduction furnace body according is disclosed, wherein the sum of all metals in the silicon carbide product and the silicon product, respectively, is less than 100 ppm. (silicon excluded) In addition the reduction furnace body comprises a second lining composed of a material which is different from the inner lining material, said second lining being arranged to contact the outer surface of the inner lining. The material of the second lining is silica stone or silica ramming paste. In particular a layer of graphite or carbon ramming paste is arranged in between the inner and the second lining, in the bottom of the body. Further, the reduction furnace body comprises a casing arranged outside the second lining. In a preferred embodiment an insulating layer is arranged in between the casing and second lining. According to an embodiment of the invention a reduction furnace body is disclosed, wherein at least one electrode, preferably composed of graphite, extends into the bottom of the body from below the body. Preferred is an electrode, preferably composed of graphite that extends into the interior of the body from above the body. Also an embodiment of the invention is a closable channel between the interior and the exterior of the body for tapping of liquid silicon is provided at the bottom of the body and, wherein the inner walls of the channel are composed of said inner lining.

The reduction furnace body may in particular possess an arrangement of one or more electric coils around the interior of the furnace body such that a magnetic field may be created in the interior of the furnace body. And, wherein the reduction furnace body is split into an upper and a lower portion, wherein the upper portion is rotatably attached to the lower portion. According to another embodiment, the sum of all metal impurities in the silicon carbide product is less than 300 ppm (w/w), particularly less than 100 ppm (w/w). According to an alternative or complementary embodiment, the sum of all metal impurities in the silicon product is less than 100 ppm (w/w), such as less than 80 ppm (w/w), preferred less than 50 ppm (w/w), more preferred less than 10 ppm (w/w).

The binding agent is capable of binding the components of the inner lining together.. Further, the inner lining composition should be stable at high temperatures, such as temperatures above 2000 °C. Resins are binding agents particularly suitable for such purposes. The resin is preferably a synthetic resin, as such resins may normally be provided with higher purity than natural resins and the resin shall not comprise elements that can contaminate the silicon product to a substantial degree. The resin may for example be a synthetic phenol-formaldehyde resin. One specific example of a synthetic resin that has given satisfactory results is Novolack. Other examples of suitable resins are Resol and Peracit

However, the inventors realize that other resins may also be used. According to one embodiment, the thermal conductivity of the inner lining material is at least 15 W/m°K at 1500 °C. The inner lining contacts the material (the reactants and the product) in the reduction furnace. However, the reduction furnace body normally comprises further parts to provide for appropriate mechanical, thermal, chemical and/or heat transfer properties.

The reduction furnace body may thus comprise a second lining arranged to contact the outer surface of the inner lining. Thus, in such an embodiment, the inner surface of the second lining is in contact with the outer surface of the inner lining. The inner and the second lining can be regarded as different protective layers. The second lining is normally composed of a heat-resistant material which is different from the inner lining material. An example of such a material is silica ramming paste or silica; stone, which may have the same purity as the quartz added as a starting material in the carbothermic reduction.

According to an embodiment of the invention there is disclosed a ladle arrangement provided with an inner lining of the composition of the invention and in particular with means tor heating, such as inductive heating means, preferred with means to bubble gas through the bottom. In particular with an arrangement for chlorine refining, during which chlorine gas, optionally together with an inert gas, such as argon, is bubbled through the liquid silicon in the ladle, in particular the gas is bubbled through the bottom of the ladle, to react with impurities, such as aluminium or calcium, to form chloride salts, e.g. AICI3 and CaCI₂. When the aluminium chlorides subsequently get in contact with air, the corresponding oxides are formed instead. The arrangement for chlorine refining preferably comprises means tor heating, such as inductive heating, of the liquid silicon. This is to ensure that the silicon remains liquid during the process. A further embodiment of the invention is therefore component of a plant, in particular a ladle and/or crucible, wherein said ladle and/or crucible comprises an inner lining characterized in that said inner lining comprises: (i) 80-95 % (w/w) of a silicon carbide product containing 1.5 ppm (w/w) or less boron and 3.0 ppm (w/w) or less phosphorus; (ii) 5-20 % (w/w) of a silicon product containing 1.5 ppm (w/w) or less boron and 3.0 ppm (w/w) or less phosphorus; and (iii) 0.1-5 % (w/w) of a reaction product of the binding agent and/or binding agent, such as a resin, e.g. a synthetic resin; and, wherein the sum of this composition is 100 % (w/w), in particular the ladle further comprises an arrangement of heating means, preferred heating means are inductive heating means. The refining may also be done in a ladle. The ladle may either be pre-heated by heating elements or by using induction.

Accordingly, there is also provided an electrode or at least a component of said electrode comprises a silicon containing material with a composition selected from

(ii) 5-20 % (w/w) of a silicon product containing 1.5 ppm (w/w) or less boron and 3.0 ppm (w/w) or less phosphorus; and in particular less than 1 ppm (w/w) of all other metals (silicon excluded) and

(iii) 0 to 5 % (w/w) of a reaction product of a binding or of a binding agent, in particular 0.1 to 5.0 % (w/w), such as a resin, e.g. a synthetic resin; and, wherein the sum of this composition is 100 % (w/w), in particular in an electrode or a component of the electrode, preferred after a heating process.

And in particular in a green electrode e.g. before a curing process, and/or tempering process the content of the binder is preferred (0.1-5 %) (w/w). Ppm (w/w) has the same meaning as ppmw (part per million by weight). The very special feature of the electrode of the invention is the ability to increase the yield of liquid silicon in an arc.

Therefore, subject matter of the invention is a component of a plant for production of liquid silicon by carbothermic reduction, in particular an inner lining of a reduction furnace body, an electrode, a green electrode, at least a component of said green electrode, a furnace electrode, particularly a furnace electrode for a reduction furnace, preferred a furnace electrode for a reduction furnace for the production of liquid silicon, more preferred a furnace electrode for a reduction furnace for the production of liquid silicon by carbothermic reduction, and/or a furnace bottom electrode, preferred the bottom electrode may be at least a part of the reaction furnace body. In a preferred embodiment the electrode is a bottom electrode, preferred a bottom electrode in an arc furnace, more preferred in an arc furnace for carbothermic reduction processes in a silicon production process. Therefore, the electrode can be used as an anode or cathode.

In embodiments of the invention the electrode or the parts of the electrode comprise the silicon containing material and are selected from a

(i) massive component of the electrode selected from electrode strand, electrode body, nipple, blocking pin, clamping jaws, heat exchanger, mounting and power connection, or

(ii) matrix for other electrode materials forming at least a part of a massive component of the electrode selected from electrode strand, electrode body, nipple, blocking pin, clamping jaws, heat exchanger, mounting and power connection, or

(iii) coating or a part of a coating for other electrode materials forming a massive component of the electrode selected from electrode strand, electrode body, nipple, blocking pin, clamping jaws, heat exchanger, mounting, power connection and a part of said components.

(iv) contact lining, sealant, adhesive and/or bonding or as a part of them, particularly at a component of said electrode selected from electrode strand, electrode body, nipple, blocking pin, clamping jaws, heat exchanger, mounting and power connection, or

(v) other component of the electrode in a matrix of other electrode materials, particular selected from graphite and silicon carbide, forming at least a component of the electrode selected from electrode strand, electrode body, nipple, blocking pin, clamping jaws, heat exchanger, mounting and power connection. In addition, the aspects (i), (ii), (iii), (iv) and/or (v) can be combined in one electrode or an electrode construction. In particular (i) and (iii), (i) and (ii), optional each in combination with (iv).

Suitable other electrode materials are in particular selected from graphite and silicon carbide. The coating may be a kind of impregnation or may be film, wherein the film or coating may have a thickness of 0.01 to 400 mm, in particular from 0.01 mm to 200 mm, preferred from 0.01 to 100 mm. Thus coating may be an outer coating of an electrode body and/or may be one of several internal and outer coating that can be supplied in a construction process. Therefore, the electrode or a component of an electrode may comprise several different layers separated by coatings of the silicon containing material. The layers may be made of the other electrode material. Such other parts in a matrix of another electrode material, in particular selected form graphite and/or silicon carbide, may comprise cylinders, polyhedron parts, conical parts, granules, pellets, bars, cylindrical bars, powders, grids, lattices, gratings, cubic parts and/or grains.

According to a further embodiment of the invention the electrode comprises (i) a silicon carbide product and a (ii) silicon product, wherein each independently contains 100 ppm (w/w) Al or less. Particular the silicon carbide product and the silicon product each contain less than 80 ppm (w/w). Preferred is that the silicon carbide product and the silicon product each contain independently less than 50 ppm (w/w), more preferred less than 10 ppm (w/w) Al, particularly less than 1 ppm (w/w) Al, and less 1 ppm (w/w) Fe, most preferred less than 0.5 ppm (w/w) Al and less 0.5 ppm (w/w) Fe. According to one embodiment of the invention the electrode or at least a component of an electrode comprise a silicon containing material containing equal or less than 1.5 ppm (w/w) silver (Ag). Wherein it is further preferred when said silicon containing material contains less 100 ppm (w/w) aluminium, particularly less than 50 ppm (w/w), preferred less than 35 ppm (w/w).

According to a preferred embodiment of the invention the electrode or the at least component of the electrode comprises a silicon containing material, wherein the sum of all other metals (excluded silicon) in the silicon carbide product and the silicon product, respectively, is less than 100 ppm (w/w), wherein particularly said other metals are selected from Ag, Al, As, Au, Ba, Be, Bi, Ca, Cd, Ce, Co, Cr, Cs, Cu, Dy, Er, Eu, Fe, Ga, Gd, Ge, Hf, Hg, Ho, In, Ir, K, La, Li, Lu, Mg, Mn, Mo, Na, Nb, No, Os, Pb, Pd, Pr, Pt, Rb, Re, Rh, Ru, Sb, Sc, Se, Sm, Sn, Ta, Tb, Te, Th, Ti, Tl, Tm, U, W, Y, Yb, Zn, Zr or wherein the sum of all of other metals is less 80 ppm (w/w), particularly less than 50 ppm (w/w), preferred less than 30 ppm (w/w), more preferred less than 10 ppm (w/w). The iron content is typically below 10 ppm (w/w). A typical method for the detection of the elements is IPC-MS, GDMS may also be used.

In one embodiment of the invention the electrode or the at least component of the electrode comprises a silicon containing material, wherein said silicon containing material contain equal or less than 1.5 ppm (w/w) silver, particularly less than 0.1 ppm (w/w) silver, particularly less than 1.0 ppm (w/w) boron (B), preferred less than 0.5 ppm (w/w), preferred less than 0.35 ppm (w/w) and/or less than 2.5 ppm (w/w) phosphorus (P), particular equal or less than 2.0 ppm (w/w) phosphorus, preferred less 0.5 ppm (w/w) P, and optional equal or less than 5 ppm (w/w) sulphur (S), particularly less than 4.5 ppm (w/w) sulphur, preferred less than 0.1 ppm (w/w) S.

In one embodiment of the invention the electrode or the at least component of the electrode comprises a silicon containing material, wherein said silicon containing material contain equal or less than

a. aluminium (Al) 30 ppm (w/w) or from 25 to 0.0001 ppt (w/w), particularly from 22 ppm (w/w) to 0.0001 ppt (w/w), and

c. calcium (Ca) 20 ppm (w/w), particularly from 15 to 0.0001 ppt (w/w), preferred from 10 ppm (w/w) to 0.0001 ppt (w/w); and optional

d. gallium (Ga) 1 ppm, particular from 0.5 ppm to 0.0001 ppt (w/w), preferred from 0.1 ppm (w/w) to 0.0001 ppt (w/w), particular preferred 1 ppb (w/w) to 0.0001 ppt (w/w),

f. phosphorus (P) 3 ppm (w/w) to 0.0001 ppt (w/w), particularly from 2 ppm (w/w) to 0.0001 ppt (w/w), preferred from 1 ppm (w/w) to 0.0001 ppt (w/w), and

g. titan (Ti) 2 ppm (w/w), particularly from 1.7 ppm (w/w) to 0.0001 ppt, preferred from 1.5 to 0.0001 ppt (w/w); and

h. zinc (Zn) 30 ppm (w/w), particularly from 25 ppm (w/w) to 0.0001 ppt (w/w) preferred from 10 ppm to 0.0001 ppt (w/w). According to another aspect of the invention the component of the plant or the silicon containing material comprises a composition comprising (i) a silicon carbide product containing silicon carbide particles, in particular as a powder and/or grain, wherein the particles ranges in particular from 0.001 pm to 10 cm, from 0.0001 mm to 50 mm, from 0.001 mm to 25 mm, from 0.0001 mm to 10 mm, preferred from 0.001 mm to 5 mm, more preferred from 0.001 mm to 2.5 mm; and optionally

(ii) a silicon product containing silicon particles, in particular as a powder and/or grains, wherein the particles range in particular from 0.001 pm to 10 cm, from 0.0001 mm to 50 mm, from 0.001 mm to 25 mm, from 0.0001 mm t 10 mm, preferred from 0.001 pm to 5 μιτι, more preferred from 10 nm to 2.5 pm, most preferred 10 nm to 0.2 pm (see Figures 4, 5 and 6).

According to a preferred embodiment of the invention the component of the plant or the silicon containing material comprises as (ii) a silicon product silicon powder, wherein the primary particles range from 10 nm to 300 nm, preferred 100 to 200 nm and the secondary particles range from 0.5 to 10 pm, preferred from 2 to 5 pm. Therefore, the silicon product comprises primary particles of 100 to 200 nm and secondary particles of 2 to 5 pm. The crystallinity of the silicon product is above 95 %. In Addition, the specific surface of the silicon product ranges from 9 to 12 m²/g, the bulk density ranges from 0.08 to 1.0 g/cm³, depending on the product. The purity of the silicon product may be according table 4 or 5:

Table 4: A composition of a component according to the invention comprises:

Table 5: A composition of a component according to the invention comprises:

'content of meta lie elements in ppm (w/w)

A preferred purity of the silicon product is 99,9999 %, metals (including iron) less than 0.5 ppma, carbon less than 10 ppma, oxygen less than 10 ppma. Purity: > 99,9999 %, metals (included iron) < 0,5 ppma, carbon < 10 ppma.

In one embodiment of the invention the component or the silicon containing material comprises a composition comprising (i) a silicon carbide product containing silicon carbide particles, wherein more than 50 wt.-% having a particle size equal or greater than 0.15 mm, particular equal or more than 25 wt.-% of said particles having a particle size equal or greater than 0.70 mm, preferred equal or more than 30 wt.-%, particularly preferred equal or more than 40 wt.-% having a particle size from about 0.71 mm to about 0.15 mm and optionally

(ii) a silicon product containing silicon particles, wherein more than 50 wt.-% having a particle size equal or greater than 0.15 mm, particular equal or more than 25 wt.-% of said particles having a particle size equal or greater than 0.70 mm, preferred equal or more than 30 wt.-%, particularly preferred equal or more than 40 wt.-% having a particle size from about 0.71 mm to about 0.15 mm. In one embodiment of the invention the green body of a component of the plant , in particular the lining of the furnace body or electrode or the at least component of the green electrode comprises a silicon containing material with a composition comprising (i) a silicon carbide product containing silicon carbide particles, wherein more than 50 wt.-% having a particle size equal or greater than 0.15 mm, particular equal or more than 25 wt.-% of said particles having a particle size equal or greater than 0.70 mm, preferred equal or more than 30 wt.-%, particularly preferred equal or more than 40 wt.-% having a particle size from about 0.71 mm to about 0.15 mm. The silicon carbide particles as well as the silicon particles are multicrystalline solids or amorphous, in particular they are independently multicrystalline. During tempering and/or heating process the domains of crystalline phases will grow e.g. will adhere to other crystalline domains. During tempering and/or heating the stability of the silicon containing material is increased by settling, growing and/or sintering processes.

In one embodiment of the invention the component of the plant, in particular the inner lining of the reduction furnace body, ladle, electrode or the at least component of the electrode comprises as said silicon product (ii) crystalline, particularly multi crystalline silicon, preferred powdered crystalline silicon, in particular said green electrode or at least a component of it. In a second embodiment of the invention the component of the plant (as heated body), in particular the inner lining of the reduction furnace body, ladle, the electrode or the at least component of the electrode comprises as said silicon product (ii) silicon nitride and optionally silicon, in particular said furnace electrode or at least a component of it. Wherein it is particularly preferred when the silicon product is at least partly reacted to silicon nitride, preferred are 5 to 100 wt.-% silicon nitride and 0 to 95 wt.% of the silicon, wherein the sum of them is 100 wt.%, particularly preferred are 20 to 100 wt.-% silicon nitride and 80 to 0 wt.-% silicon, more preferred 50 to 100 wt.-% silicon nitride and 50 to 0 wt.-% silicon, most preferred 80 to 100 wt.-% silicon nitride and 20 to 0 wt.-% silicon. According to a particularly preferred embodiment of the invention the silicon product, in particular in the component of the plant, in particular the inner lining of the reduction furnace body, ladle, furnace electrode or at least a component of it, comprises 90 to 100 wt.- % of silicon nitride and 10 to 0 wt.-% silicon, 95 to 100 wt.-% silicon nitride and 5 to 100 wt.-% silicon, 98 to 100 wt.-% silicon nitride and 2 to 0 wt.-% silicon, in particularly with the aforementioned purities.

Further, in one embodiment of the invention the electrode or the at least component of the electrode comprises a layer of graphite or carbon ramming paste that is arranged in between the nipple and the electrode.

According to a further aspect of the invention the component of the plant, in particular the inner lining of the reduction furnace body, ladle, the electrode or at least a component of the electrode, in particular the green electrode or the at least component of the green electrode, comprises a silicon containing material with a composition comprising a binding agent, wherein the binding agent is a resin selected from synthetic phenol- formaldehyde resins, or the binding agent is a binding agent known as a binding agent for pharmaceutical formulations. In particular the binding agent may be selected from carbohydrates or polyvinylpyrrolidone. The carbohydrates may be selected from saccharides, disaccharides, monosaccharides, cellulose, hydroxypropylmethylcellulose (HPMC), methylcellulose.in particular sucrose, lactose, mannitol, dectrin, dextrose, microcrystalline cellulose, cellulose ether, starch. Further, the component of the plant, in particular the inner lining of the reduction furnace body, ladle, the electrode or at least a component of the electrode need to stand the high temperatures of the carbothermic reduction, and the inventors have chosen the components of the electrode or at least a component of the electrode, and their proportions, accordingly. Thus, (i) 80-95 % (w/w), preferably 85-95 % (w/w), more preferably 87-93 % (w/w) of the component or of the silicon containing material of the electrode or at least a component of the electrode are constituted by the carbide product. The carbide product contains 1.5 ppm (w/w) or less, preferably 1.0 ppm (w/w) or less, more preferably 0.5 ppm (w/w), less than 0.35 ppm (w/w) boron or less, most preferably 0.2. ppm (w/w) or less B, preferred 0.1 ppm (w/w) to 0.0001 ppt (w/w). Further, the carbide product contains 3.0 ppm (w/w) or less, preferably 2.0 ppm (w/w) or less, more preferably 1.0 ppm (w/w) or less, most preferably 0.5 ppm (w/w) or less phosphorus, preferred 0.1 ppm (w/w) to 0.0001 ppt (w/w). Sum of composition is 100 % (w/w). Further, 5-20 % (w/w), preferably 5-15 % (w/w), more preferably 7-13 % (w/w) of the silicon containing product of the component of the plant, in particular the inner lining of the reduction furnace body, ladle, the electrode or at least a component of the electrode is constituted by the silicon product. The (ii) silicon product contains 1.5 ppm (w/w) or less, preferably 1.0 ppm (w/w) or less, more preferably 0.7 ppm (w/w) or less, most preferably 0.5 ppm (w/w), preferably 0.35 ppm (w/w) or less boron, preferred 0.1 ppm (w/w) to 0.0001 ppt (w/w) boron. Further, the silicon product contains 3.0 ppm (w/w) or less, preferably 2.0 ppm (w/w) or less, more preferably 1.0 ppm (w/w) or less, most preferably 0.5 ppm (w/w) or less phosphorus, less than 0.2 ppm (w/w), preferred 0.1 ppm (w/w) to 0.0001 ppt (w/w).

According to one embodiment, the silicon carbide product of the silicon containing material of the component of the plant, in particular the inner lining of the reduction furnace body, ladle, the electrode or at least a component of the electrode contains less than 100 ppm (w/w) Al, in particular less than 50 ppm (w/w), preferred less than 25 ppm (w/w), less than 20 ppm (w/w), less than 15 ppm (w/w) or less. According to an alternative or complementary embodiment, the silicon product of the silicon containing material of the electrode or at least a component of the electrode contains 200 ppm (w/w) Al or less, such as 100 ppm (w/w) Al or less, in particular 50 ppm (w/w) or less, preferred 25 ppm (w/w) or less, 20 ppm (w/w) or less, 15 ppm (w/w) or less.

The binding agent is capable of binding the components of the inner lining together. Further, the components such as the reduction furnace body and/or electrode or parts of the electrode composition should be stable at high temperatures, such as temperatures above 2000 °C. Resins are binding agents particularly suitable for such purposes. The resin is preferably a synthetic resin, as such resins may normally be provided with higher purity than natural resins and the resin shall not comprise elements that can contaminate the silicon product to a substantial degree. The resin may for example be a synthetic phenol-formaldehyde resin. One specific example of a synthetic resin that has given satisfactory results is Novolack. Other examples of suitable resins are Resol and Peracit. However, the inventors realize that other resins may also be used. According to one embodiment, the thermal conductivity of the electrode or parts of the electrode is at least at 15 W/m°K (Kelvin) at 1500 °C, preferred are thermal conductivities lower than 12 W/m°K at temperature above 900 °C.

A particular aspect of the invention is a process for the production of a component of a plant for production of silicon by carbothermic reduction, wherein a green component comprising (i) 80-95 % (w/w) of a silicon carbide product containing 1.5 ppm (w/w) or less boron and 3.0 ppm (w/w) or less phosphorus; (ii) 5-20 % (w/w) of a silicon product containing 1.5 ppm (w/w) or less boron and 3.0 ppm (w/w) or less phosphorus; and (iii) 0.1-5 % (w/w) of a binding agent, such as a resin, e.g. a synthetic resin; and, wherein the sum of this composition is 100 % (w/w), is formed and optionally cured.

A preferred process comprises the steps of

(i) curing of the formed green component, and optional

(ii) tempering of the green component and optional

(iii) heating of the green component, wherein the temperature of step (ii) is higher than in step (i) and the temperature in step (iii) is higher than in step (ii).

Particularly the curing, tempering and/or heating is performed in an atmosphere of an inert gas as argon. Further the curing, tempering and/or heating may performed in an atmosphere containing nitrogen, particular the heating step is performed in an nitrogen containing atmosphere, preferred in a nitrogen atmosphere, e.g. from 0.01 to 100 Vol.-%, in particular from 10 to 100 Vol.-% N₂ ad 100 Vol.-% of inertgas particular argon, preferred from 30 to 100 Vol.-% N2 ad 100 Vol.-% of inertgas. During the process the silicon reacts with the nitrogen and one product of the reaction is silicon nitride S13N4. In a preferred embodiment of the invention nearly the entire silicon product becomes silicon nitride. During the tempering and/or heating process silicon carbide and silicon nitride may grow together and adhere. This step is particularly relevant for the later stability of the component. Preferred are green components that are formed of a composition comprising

(i) 85-95 % (w/w) of a silicon carbide product containing 1.5 ppm (w/w) or less boron and 3.0 ppm (w/w) or less phosphorus, each preferred less than 0.2 ppm (w/w); (ii) 5- 15 % (w/w) of a silicon product containing 1.5 ppm (w/w) or less boron and 3.0 ppm (w/w) or less phosphorus, each preferred less than 0.2 ppm (w/w); and

(iii) 0.1-2 % (w/w) of a binding agent, such as a resin, e.g. a synthetic resin; and, wherein the sum of this composition is 100 % (w/w).

According to one further embodiment of the process of the invention the temperature during the several steps is controlled. The process parameters are delicate. Preferred is when the temperature (i) during the curing step is from 0 to 300 °C, in particular form 50 to 300 °C, preferred from 100 to 250 °C, particularly preferred from 150 to 250 °C, each plus/minus 25 °C, preferred plus/minus 5 °C, and optional (ii) during the tempering step from 200 to 500 °C, in particular from 200 to 450 °C, preferred from 250 to 400 °C each plus/minus 25 °C, preferred plus/minus 5 °C, and optional (iii) during the heating step from 350 to 1400 °C, in particular in several steps from 400 to 600 °C or from 600 °C to 1400 °C, or from 600 to 900 °C and from 900 to 1400 °C, in particular wherein the temperature setting is +5 °C/hour, in particular + 10 °C/hour or + 15 °C/hour in step (i), (ii) and/or (iii).

Another process parameter is the time frame of the steps. In particular the holding time in each step of (i), (ii) and/or (iii) independently is from 10 to 250 hours, in particular the holding time per step is from 25 to 150 hours, preferred from 50 to 100 hours, in particular in step (i) and (ii). In particular the process may be run with temperature ramps of +5 °C/hour, + 10 °C/hour, + 15°C/hour, 20 °C/hour, 25 °C/hour or 30 °C/hour, wherein the temperature ramps from +1 to 15 °C/hour are preferred due to decomposition processes of the binding agent. Therefore, preferred the temperature is increased with +5 °C/hour or most preferred by 10 °C/hour. During the late heating process + 15 °C/hour is also possible. A typical process of the invention will be as shown in the following table 6:

Typical green components of the plant are selected from the inner lining of a reduction furnace body, inner lining of SiC furnace, inner lining of heating or drying oven, inner lining of pelletizing, tabletting, briquetting equipment, inner lining of extruder, electrode component, electrode, tapping channel, supply channel, component of tapping equipment, ladle, crucible, inner lining of holding furnace, inner lining of directional solidification furnace, lining of devices that are in contact with quartz, silicon carbide, silicon products or formulation comprising any of quartz, silicon carbide or silicon product, inner lining of supply channels and inner lining of supply pipes.

One further embodiment of the invention is a process for the production of an inner lining of a reduction furnace and/or electrode or of at least a component of said electrode, or an electrode or a component of the electrode, such as electrode, bottom electrode, electrode strand, electrode body, electrode nipple, electrode blocking pin, electrode clamping jaws and/or electrode mounting or green body of any of these components obtained by the process of the invention, wherein a green electrode or at least a component of said green electrode is formed and optionally cured. In particular the process of the invention comprises the steps of

(i) curing of the formed green electrode or of at least a component of said green electrode, and optional

(ii) tempering of the green electrode or of at least a component of said green electrode, in particular cured green electrode, and optional

(iii) heating of the green electrode or of at least a component of said green electrode, in particular tempered electrode, wherein the temperature of step (ii) is higher than in step (i) and the temperature in step (iii) is higher than in step (ii). According to one embodiment of the invention it is further preferred, that a component of the green electrode is an electrode strand, electrode body, nipple, blocking pin, clamping jaws, heat exchanger, mounting, power connection or a part of aforementioned components. Hence it is an embodiment of the process of the invention, wherein the curing, tempering and/or heating is performed in an atmosphere of an inertgas as argon.

According to the process of the invention the curing, tempering and/or heating is performed in an atmosphere containing nitrogen, in particular the heating step is performed in an nitrogen containing atmosphere, preferred in a nitrogen atmosphere, e.g. from 10 to 100 Vol.-% N₂ ad 100 Vol.-% of inertgas particular argon. During the process the silicon reacts with the nitrogen and one product of the reaction is silicon nitride S13N4. In a preferred embodiment of the invention nearly the entire silicon product becomes silicon nitride. During the tempering and/or heating process silicon carbide and silicon nitride may grow together and adhere. This step is particularly relevant for the later stability of the electrode.

Further, the process of the invention has a step, wherein the green component of the plant for production of liquid silicon by carbothermic reduction, in particular an inner lining of furnace body and/or electrode or a component of the green electrode is formed of a silicon containing material with a composition selected from

(i) 80-95 % (w/w) of a silicon carbide product containing 1.0 ppm (w/w) or less boron and 1.0 ppm (w/w) or less phosphorus; (ii) 5-20 % (w/w) of a silicon product, in particular silicon, preferred silicon powder, containing 1.0 ppm (w/w) or less boron in particular less than 0.5 ppm ( w/w) to 0.1 ppt (w/w) boron, and 1.0 ppm (w/w) or less phosphorus, in particular less than 0.5 ppm ( w/w) to 0.1 ppt (w/w); and

(iii) 0.1-5 % (w/w) of a binding agent, such as a resin, e.g. a synthetic resin; and, wherein the sum of this composition is 100 % (w/w).

The process steps (i) and (ii) are especially critical and a reduced heating rate is recommended. It should also be envisaged a holding time for these steps. Step (iii) is probably not quite as critical, but somewhat reduced speed should be selected. It is very important that the silicon containing material paste, in particular extrudable mass, is protected from oxidation during this process. It is recommended to fill argon in the furnace so that the atmosphere is reducing. The furnace should of course be as close as possible. It is recommended to switch from argon to nitrogen as the inert gas from 900 - 1000 °C (degrees) and up to 1400 °C to get a solid bound nitride connection (S13N4) between added Si-metal and nitrogen. In particular nitrogen is used during the whole heating process. The silicon containing material (synonym to composition) is an extrudable mass or capable of continuous casting, in particular the silicon containing material is compactable, in particular the material is compactable by a factor from 0.1 to 4, preferred from 1 to 2.5, particularly preferred about a factor of 2.4 (in one dimension, wherein the other two dimension remain, see examples).

Table 6: (i) curing step, (ii) tempering step and (iii) heating step

Wherein the aforementioned times directly depend from the component or of the electrode that will be made, in particular the holding time will depend from the dimensions of the parts. Therefore, the time for the production of the electrode body, nipple may be much longer than for a blocking pin.

A further embodiment of the invention is an electrode, bottom electrode, electrode strand, electrode body, electrode nipple, electrode blocking pin, electrode clamping jaws and/or electrode mounting, and according to an alternative a green bodies of electrode, green bottom electrode, electrode strand, electrode body, electrode nipple, electrode blocking pin, electrode clamping jaws and/or electrode mounting obtained by a process of the invention. An aspect of the invention is the use of a silicon containing material (synonym with component) with a composition selected from

(i) 80-95 % (w/w) of a silicon carbide product containing 1.5 ppm (w/w) or less boron, in particular less than 0.5 ppm (w/w) to 0.1 ppt (w/w) boron, and 1.0 ppm (w/w) or less phosphorus, in particular less than 0.5 ppm ( w/w) to 0.1 ppt (w/w);

(ii) 5-20 % (w/w) of a silicon product containing 1.5 ppm (w/w) or less boron, in particular less than 0.5 ppm (w/w) to 0.1 ppt (w/w) boron, and 3.0 ppm (w/w) or less phosphorus, in particular less than 0.5 ppm ( w/w) to 0.1 ppt (w/w); preferred a silicon powder, and

(iii) 0 to 5 % (w/w), in particular 0.1-5 % (w/w), of a binding agent, such as a resin or carbohydrate, e.g. a synthetic resin and/or saccharose; and,

wherein the sum of this composition is 100 % (w/w) for the production of an electrode or a component of an electrode, in particular for the production of a

(iv) massive component of the electrode selected from electrode strand, electrode body, nipple, blocking pin, clamping jaws, heat exchanger, mounting, power connection, or

(v) matrix for other electrode materials, particularly selected from graphite and/or silicon carbide, forming at least a part of a massive component of the electrode selected from electrode strand, electrode body, nipple, blocking pin, clamping jaws, heat exchanger, mounting and power connection, or

(vi) coating or a part of a coating for other electrode materials forming a massive component of the electrode selected from electrode strand, electrode body, nipple, blocking pin, clamping jaws, heat exchanger, mounting and power connection, or a part of said components, or

(iv) contact lining, sealant, adhesive and/or bonding or as a part of them, in particular at a component of said electrode selected from electrode strand, electrode body, nipple, blocking pin, clamping jaws, heat exchanger, mounting and power connection, or

(vii) other component of the electrode in a matrix of other electrode materials, in particular selected from graphite and silicon carbide, forming at least a component of a of the electrode selected from electrode strand, electrode body, nipple, blocking pin, clamping jaws, heat exchanger, mounting and power connection.

Accordingly, the electrode of the invention is used together with a reaction furnace body for production of liquid silicon by carbothermic reduction, said body comprising an inner lining, characterized in that inner said lining comprises: 80-95 % (w/w) of a silicon carbide product containing 1.5 ppm (w/w) or less boron and 1.0 ppm (w/w) or less phosphorus; 5-20 % (w/w) of a silicon product containing 1.5 ppm (w/w) or less boron, in particular less than 0.5 ppm (w/w) to 0.1 ppt (w/w) boron, and 1.0 ppm (w/w) or less phosphorus, in particular less than 0.5 ppm (w/w) to 0.1 ppt (w/w); and 0-5 % (w/w) of a reaction product of a binding agent, such as a resin, e.g. a synthetic resin.

According to one embodiment the reduction furnace and/or electrode is used for production of a silicon product containing 1.5 ppm (w/w) or less boron and 3.0 ppm (w/w) or less phosphorus by carbothermic reduction, comprising the steps of:

a) adding a silicon dioxide, silica, precipitated silica, pyrogenic silica, and/or quartz product containing 1.5 ppm (w/w) or less boron and 3.0 ppm (w/w) or less phosphorus, in particular 0.50 ppm (w/w) or less boron and 1.0 ppm (w/w) or less phosphorus and carbon-source comprising a silicon carbide product, carbohydrates, sucrose, sugar and/or a mixture of at least two of the, in particular a mixture of SiC and sugar, preferred a silicon carbide product or sugar containing 1.5 ppm (w/w) or less boron and 3.0 ppm (w/w) or less phosphorus; to a reduction furnace having at least one electrode; and

b) heating the mixture such that a melt of the silicon product is formed, wherein said at least one electrode or a component of said at least one electrode comprise a silicon containing material with a composition selected from: (i) 80-95 % (w/w) of a silicon carbide product having at least the same purity as the silicon carbide product added in step a), (ii) 5-20 % (w/w) of a silicon product having at least the purity of the silicon product produced by the method; and (iii) 0-5 % (w/w) of a reaction product of a binding agent, such as a resin and/or carbohydrate.

Further embodiments of the invention are selected from components obtained according to the method of the invention, such as the inner lining of a reduction furnace body, inner lining of SiC furnace, inner lining of heating or drying oven, inner lining of pelletizing, tabletting, briquetting equipment, inner lining of extruder, electrode component, electrode, tapping channel, supply channel, component of tapping equipment, ladle, crucible, inner lining of holding furnace, inner lining of directional solidification furnace, lining of devices that are in contact with quartz, silicon carbide, silicon products or formulation comprising any of quartz, silicon carbide or silicon product, inner lining of supply channels and inner lining of supply pipes, from electrode strand, electrode body, nipple, blocking pin, clamping jaws, mounting or green body of any of them or electrode, such as bottom electrode or electrode assembly comprising at least two of the aforementioned electrode components.

An additional embodiment is the use of a component of a plant for production of liquid silicon by carbothermic reduction for production of a silicon product containing 1.5 ppm (w/w) or less boron and 3.0 ppm (w/w) or less phosphorus by carbothermic reduction, comprising the steps of:

a) adding a silicon dioxide product selected from silica and quartz product containing 1.5 ppm (w/w) or less boron and 3.0 ppm (w/w) or less phosphorus, in particular, 0.50 ppm (w/w) or less boron and 1.0 ppm (w/w) or less phosphorus and a carbon source comprising silicon carbide product, carbohydrate and/or sucrose containing 1.5 ppm (w/w) or less boron and 3.0 ppm (w/w) or less phosphorus; to a reduction furnace having at least one electrode; and

b) heating the mixture such that a melt of the silicon product is formed, wherein said reduction furnace and/or is a component of any of claims 1 to 30 comprising a composition selected from:

(i) 80-95 % (w/w) of a silicon carbide product having at least the same purity as the silicon carbide product added in step a), (ii) 5-20 % (w/w) of a silicon product, in particular comprising silicon nitride and optionally silicon, having at least the purity of the silicon product produced by step a) and b); and (iii) 0-5 % (w/w) of a reaction product of a binding agent, such as a resin or carbohydrate.

A further embodiment is a method for production of a silicon product containing 1.5 ppm (w/w) or less boron and 3.0 ppm (w/w) or less phosphorus by carbothermic reduction, comprising the steps of: a) adding a quartz product containing 0.50 ppm (w/w) or less boron and 1.0 ppm (w/w) or less phosphorus and a silicon carbide product containing 1.5 ppm (w/w) or less boron and 3.0 ppm (w/w) or less phosphorus; to a reduction furnace having an inner lining; and b) heating the mixture such that a melt of the silicon product is formed, wherein said lining comprises: 80-95 % (w/w) of a silicon carbide product having the same purity as the silicon carbide product added in step a); 5-20 % (w/w) of a silicon product having at least the purity of the silicon product produced by the method; and 0.1-5 % (w/w) of a binding agent or a reaction product of a binding agent, such as a resin. Also, there is provided a method for production of a silicon product containing 3.0 ppm (w/w), preferred 1.5 ppm (w/w) or less boron and 5.0 ppm (w/w), preferred 3.0 ppm (w/w) or less phosphorus by carbothermic reduction, comprising the steps of: a) adding a silicon dioxide, silica, precipitated silica, pyrogenic silica, and/or quartz product or a mixture of at least two of them containing 0.50 ppm (w/w) or less boron and 1.0 ppm (w/w) or less phosphorus and a carbon-source comprising silicon carbide product, carbohydrates, sucrose, sugar and/or a mixture of at least two of the, in particular a mixture of SiC and sugar, preferred a silicon carbide product or sugar, containing 1.5 ppm (w/w) or less boron and 3.0 ppm (w/w) or less phosphorus; to a reduction furnace having an electrode; and b) heating the mixture such that a melt of the silicon product is formed, wherein said electrode or component of the electrode comprise a silicon containing material with a composition selected from: (i) 80-95 % (w/w) of a silicon carbide product containing 1.5 ppm (w/w) or less boron and 3.0 ppm (w/w) or less phosphorus (ii) 5-20 % (w/w) of a silicon product, in particular silicon nitride and optionally silicon, containing 1.5 ppm (w/w) or less boron, in particular less than 0.5 ppm ( w/w) to 0.1 ppt (w/w) boron, and 3.0 ppm (w/w) or less phosphorus, in particular less than 0.5 ppm (w/w) to 0.1 ppt (w/w) phosphorus; and 0-5 % (w/w) of a binding agent or a reaction product of the binding agent, preferred are 0 % binding agent, such as a resin and 0 to 5 % (w/w) reaction product of the binding agent.

The invention will be described in more detail according to the following figures and general executive examples without limiting the invention to these examples.

Drawings

Figure 1 shows an embodiment of a whole system for producing solar grade silicon with an electrode of the invention 113 (bottom electrode not shown in picture).

Figure 2 shows„an embodiment of a reduction furnace body for carrying out a carbothermic reduction of prepared quartz material to silicon, with an electrode of the invention 205 (bottom electrode not shown)

Figure 3 shows an industrial setting of a reduction furnace body, electrode 305.

Figure 4 shows a TEM-image of silicon powder: primary particles

Figure 5 shows a REM-image of silicon powder: secondary particles

Figure 6 shows a TEM-image of silicon powder: crystalline part Detailed Description of the Invention

As a first aspect of the present disclosure, there is thus provided an electrode or a component of an electrode for production of liquid silicon by carbothermic reduction, said electrode or component of the electrode comprising a silicon containing product, characterized in that said silicon containing product comprises 80-95 % (w/w) of a silicon carbide product containing 1.5 ppm (w/w) or less boron and 3.0 ppm (w/w) or less phosphorus; 5-20 % (w/w) of a silicon product containing 1.5 ppm (w/w) or less boron and 3.0 ppm (w/w) or less phosphorus; and 0 to 6 % (w/w) reactants of the binding agent, or in the green electrode or green parts of the electrode 0.1-5 % (w/w) of a binding agent, such as a resin, e.g. a synthetic phenol formaldehyde resin.

The liquid silicon produced in the reduction furnace and the electrode according to the invention may be refined by gas extraction with chlorine gas, which primarily removes aluminium (Al), calcium (Ca) and other elements capable of forming salts or slag with the chlorine gas under the prevailing conditions. Boron (B) and phosphorus (P) are not such elements. The gas extraction may be followed by refinement by directional solidification, during which iron (Fe), Al and other metals having a low distribution coefficient are separated from the solidifying silicon since they have a higher solubility in the melt. However, the distribution coefficients of B and P are relatively high.

To compensate for the inefficient removal of B and P in the refining steps described above, starting materials having low levels of the elements in question are selected. Further, it is important that the levels are not increased during the carbothermic reduction. The inventors have chosen the upper limits of B and P in the components for the inner lining of the reduction furnace and for the electrode as well as for parts of it, in particular for all parts that are in direct contact with the silicon melt or through diffusion of impurities, accordingly.

The energy required for the carbothermic reduction is preferably provided by a power source, such as a DC or an AC power source, connected to at least two electrodes arranged such that an electric arc can provide heat material inside the furnace.

Thus, according to one embodiment, a (first) electrode according to the invention, in particular an electrode made of the silicon containing material, is arranged to extend into the interior of the furnace from above the furnace. The furnace may for example have an open top, and the (first) electrode may extend through the opening. In an embodiment, more than one, such as two or three (first) electrodes are arranged to extend into the interior of the furnace from above the furnace. According to a further embodiment, one or more (second) electrode(s) is/are arranged such that it/they extend(s) into the bottom of the furnace from below the furnace, in particular according to the invention, in particular an electrode made of the silicon containing material. Such (a) (second) electrode(s) do/does not extend into the interior of the furnace where it/they can contact the material provided therein. Rather, its/their one end is embedded in the bottom of the furnace (see e.g. fig 2, 206 and fig. 3, 306). According to one aspect of the invention the bottom electrode may be made of graphite and the inner lining of the reduction furnace is made of the silicon containing material (component) and therefore, possesses the function of a bottom electrode with an increased surface area compared with the originally bottom electrode. In this case the function of graphite electrode is more or less the function of a contacting element.

The first and/or the second electrode(s) is/are preferably composed of the silicon containing material or at least a part of them is composed of the silicon containing material. The high purity silicon containing material optionally in combination with other electrode materials, such as high purity graphite that does not contaminate the liquid silicon is available for such applications. In one embodiment, a layer or a block of graphite or carbon ramming paste is arranged between the inner lining and the second lining in the bottom of the furnace (see e.g. fig 2, 203) Preferably, such layer or block of graphite or carbon ramming paste is in contact with an electrode, for example, the upper end of the second electrode(s) may be embedded in the layer or block of graphite or carbon ramming paste (see e.g. fig 2, 203 and 206). One of the effects of the layer or block of graphite or carbon ramming paste is thus an increase of the surface area of the electrode.

In one embodiment, the reduction furnace body comprises a casing, for 25 holding the furnace construction together. The casing may for example be the outermost layer of the furnace construction (see e.g. fig 2, 204). Thus, if the second lining is used, the casing is preferably arranged outside the second lining. The casing may for example be composed of steel, preferably nonmagnetic steel. The furnace may be equipped with a rotating upper part (split 30 body) that may rotate independently from the lower part of the furnace. The split body construction may improve the material transport and gas permeability in the furnace. Furthermore, one or more electric coils may be arranged around the furnace interior to create magnetic fields to rotate the electric arcs inside the furnace. Such a construction will distribute the energy more evenly in the reaction zone.

Further, an insulating layer, such as a web or a fabric may be arranged between the casing and second lining. In addition providing an insulating effect, such a layer may facilitate movements of the linings relative to the casing. Such movements may for example be the result of temperature changes.

Normally, starting materials are fed to the reduction furnace through its open top. Further, the silicon liquid produced accumulates at the bottom of the reduction furnace. Therefore, a tapping channel/runner is preferably provided at the bottom of the furnace. Through such a channel/runner, the silicon liquid produced is emptied into another compartment, such as a ladle, for further processing (e.g. refining) (see e.g. fig 3, 302). The channel is preferably closable such that the tapping may be controlled.

Such a channel/runner penetrates all layers of the furnace construction. The inner wall(s) of the channel/runner is/are however preferably composed of said inner lining in order to prevent contamination of the silicon liquid flowing through the channel/runner. The reduction furnace is normally equipped with a forced cooling arrangement. For example, the reduction furnace may be cooled from below, e.g. by air or another cooling media, such as another gas. The inventors have found that a sufficient cooling effect may be obtained if the thermal conductivity through the bottom of the furnace is higher than 9.5 W/°K (the combined value for the thermal conductivity through ah layers of the bottom of the furnace).

As a second aspect of the present disclosure, there is provided a method for production of a silicon product containing 1.5 ppm (w/w) or less boron, particularly less than 1.0 ppm (w/w), preferred less than 0.2 ppm (w/w) boron, and 3.0 ppm (w/w) or less phosphorus, particularly less than 1.0 ppm (w/w), preferred less than 0.1 ppm (w/w) phosphorus, by carbothermic reduction. The method comprises the steps of: a) adding a silicon dioxide, a silica and/or quartz product containing 0.5 ppm (w/w) or less boron and 1.0 ppm (w/w) or less phosphorus and a carbon-source, a silicon carbide product and/or carbohydrate, such as sucrose and/or sugar; containing 1.5 ppm (w/w) or less boron and 3.0 ppm (w/w) or less phosphorus to a reduction furnace having an inner lining and an electrode; and b) heating the mixture such that a melt of the silicon product is formed, wherein said electrode, or electrode and lining comprises silicon containing material containing: (i) 80-95 % (w/w) of a silicon carbide product containing 1.5 ppm (w/w) or less boron and 3.0 ppm (w/w) or less phosphorus; (ii) 5-20 % (w/w) of a silicon product, in particular silicon nitride, containing 0.5 ppm (w/w) or less boron and 1.0 ppm (w/w) or less phosphorus, particularly less than 1.0 ppm (w/w), preferred less than 0.1 ppm (w/w) phosphorus; and 0 to 5 % (w/w) in particular 0 %(w/w) binding agent, alternative 0.1-5 % (w/w) of a binding agent or its reactions products.

In one embodiment, the silicon product produced by the method contains 1.0 ppm (w/w) or less boron and/or 2.0 ppm (w/w) or less phosphorus. A few further embodiments of some aspects are discussed in some detail below. The silicon carbide product in the electrode and/or the inner lining may for example be of the same quality as the silicon carbide product added in step a). Thus, the silicon carbide product of the electrode and/or the inner lining may contain 1.5 ppm (w/w) or less boron and 3.0 ppm (w/w) or less phosphorus. A production of such silicon carbide is briefly explained in the example below referring to figure 1. Thus, a single production process may be used for providing silicon carbide for two different, but related, applications. The silicon product in the inner lining may be a refined version of the silicon product produced by the method (such refinement is discussed above). In particular the silicon product can by furnished by JSSi GmbH Evonik Industries AG and SolarWorld as SUNSil®. Thus, the silicon product of the electrode, at least a component of the electrode and/or the inner lining may contain 0.6 ppm (w/w) or less boron and 0.8 ppm (w/w) or less phosphorus. Accordingly, the method may be used in the production of one of the aforementioned components of the reduction furnace employed in the method. The mixture is heated to at least 2000 °C in step b). The furnace may be cooled such that the 1410 °C isotherm is positioned to a pre decided position in the inner lining.

Examples

An exemplary system for production of solar grade silicon is described below with reference to figure 1. Natural quartz 101 from a mine is added to quartz: processing plant 102, in which the natural quartz undergoes one or more of the steps of crushing, screening, milling, optical sorting, magnet separation, gravimetric separation, flotation and acidic treatment optical sorting comprises optical detection of impurities in the natural quartz. For example, the impurities may be visible as dark spots. Further, the optical sorting may comprise the removal of the pieces of material on which such spots have been detected by means of compressed air. That is, such pieces may be blown off a transport belt, such as a conveyor belt or the like. The acidic treatment may comprise dissolving and removing impurities. In the flotation pieces of material having different physiochemical surface properties than quartz are separated. The magnet separation comprises the removal of pieces of material containing magnetic minerals.

Some of the quartz 103 from the quartz processing plant 102 is added to a silicon carbide production plant 105, to which carbon black 104 of high 30 purity is also added. Such carbon black may for example be derived from natural gas. In an alternative high purity silicon dioxide, e.g. greater or equal to 99.9999 weight.-%, sum of all metals <0.005 ppm (w/w) silicon excluded such as silica is furnished to the processing plant 102. The same type of processed quartz 106 (or in an alternative silicon dioxide, silica) from the quartz processing plant 102 is added, together with recycled silicon metal 107 from downstream positions in the system and silicon carbide 108 from the silicon carbide production plant 105, to an arrangement for weighing and mixing 109. The appropriately mixed material is then added to a reduction furnace 111 by means of charging equipment 110. The weight ratio 5 of quartz, to silicon carbide in the material added to the reduction furnace 111 is about 1.1 :1. A smoke hood 112 is arranged above the reduction furnace 111 to collect gases formed during the reduction process. The collected gases are routed, via a duct 116, to a device for flue gas filtering and dust treatment 117. The device 117 comprises a chimney 118 through which the 10 purified gases are expelled.

A first electrode 113, composed of the silicon containing material, in particular of the composition of the invention, is arranged to extend into the reduction furnace 111 from above. A second electrode 114, also composed of the silicon containing material of the invention, is arranged in the bottom of the reduction furnace. The first 113 and the second 114 electrode are connected to a power source 115, such as a DC or an AC power source, for generating an electric arc that heats the carbothermic reduction process. The silicon melt formed in the reduction furnace 114 is tapped through a tapping zone of the reduction furnace 114 into ladle 119, which has been preheated by a ladle preheating device 135. During, the tapping, an inert gas, such as argon, may be bubbled through the ladle to sir the silicon melt therein. Material may be continuously added to the reduction furnace 111. The reduction furnace 111 optionally has a split body, wherein the upper portion may rotate independently of the lower portion. The filled ladle 119 is moved to an arrangement for chlorine refining, during which chlorine gas, optionally together with an inert gas, such as argon, is bubbled through the liquid silicon in the ladle 119 to react with impurities, such as aluminium or calcium, to form chloride salts, e.g. AICI3 and CaC . When the chlorides subsequently get in contact with air, the corresponding oxides are formed instead. Also, chlorine gas is formed again. The gases are then purified in a gas cleaning system 121 and preferably routed to the chimney 118 of the device for flue gas filtering and dust treatment 117. The arrangement for chlorine refining preferably comprises means for heating 120, such as inductive heating, of the liquid silicon. This is to ensure that the silicon remains liquid during the process.

The refined silicon melt is then, optionally after filtering, added to a preferably preheated crucible/mould 122 on a device for directional solidification in filing position 123. The refined silicon may also be stored in an induction furnace or holding furnace for some time before being added to the preheated crucible 122. The device comprises a furnace 124 having heating elements 125. When the crucible/mould 122 has been filled, the device for directional solidification is set to its processing position 126, in 10 which the crucible/mould 122 is placed in the furnace 124. During the processing, the crucible/mould 122 is cooled from below to generate a bottom-up solidification, which results in a concentration of impurities in the top layer of the formed solid silicon ingot. The crucible/mould 122 is then removed from the ingot in a crucible/mould removal step 127. This normally involves breaking the crucible/ mould 122 to release the ingot. The ingot is then cut to remove the outermost layer of its sides and bottom as well as the impurities-containing top layer in an ingot cutting step 128. The ingot may then be subjected to the steps of etching and washing 129 packing 130 and storage 131 , before it is 20 transported to a customer, which may recrystallize the ingot and then cut it to wafers for the production of solar cell panels.

All or some of the side and bottom layers removed in the ingot cutting step 128 are recycled to the process, preferably to the arrangement for weighing and mixing 109, after a step of milling, sandblasting and/or crushing 132. The top layer from the ingot cutting step 128 is wasted to prevent accumulation of impurities in the system 100. The milling, sandblasting and/or crushing step 132 also produces some waste.

An exemplary embodiment of a reduction furnace for carrying out a carbothermic reduction of quartz is described below with reference to figure 2. The reduction furnace 200 has an open top, through which starting materials (prepared silicon dioxide, in particular silica and/or quartz, carbon-source, in particular silicon carbide and/or carbohydrates and optionally, recycled silicon metal) are added. The reduction furnace 200 comprises a second/middle lining 201 , preferably composed of the same type of prepared quartz as is added as a starting material. The reduction furnace 200 in particular further comprises an inner lining 202 composed of a claimed composition, e.g. a mixture of high-purity silicon carbide (the same as is added as a starting material), high-purity silicon (in particular the same as is produced by the system of figure 1 ) and a high-purity synthetic resin, in the bottom of the reduction furnace 200, a block 203 of graphite or carbon ramming paste is arranged between the middle lining 201 and the inner lining 202. In particular the inner lining is composed of the claimed composition. The reduction furnace 200 is held together by a casing 204, which may be composed of steel. Optionally the furnace may be constructed as a split body with an upper ring rotating independently from the lower part. The 10 casing 204 is normally arranged outside of the middle lining 201. Further, an insulating layer 207, such as an insulating fabric, may be arranged between the casing 204 and the middle lining 201. Such an insulating layer 207 is normally thin. Preferably, it has a thickness of less than 10 mm, such as about 5 mm. If the insulating layer 207 is too thick, the heat transport through the bottom and/or sides of the reduction furnace becomes too low for an efficient cooling of the middle lining. One purpose of arranging the insulating layer 207 between the middle lining 201 and the casing 204 is to facilitate movements of the linings 201 , 202 relative the casing. Such movements are normally caused by expansions or contractions of the lining materials during temperature changes. A first electrode 205 or a set of first electrodes composed of the silicon containing material of the invention extends into the reduction furnace 200 interior from above. Further, a second electrode 208 extends through the casing 204 and the middle lining 201 and into the block 203 from below composed of the silicon containing material of the invention. The electrodes 205, 206 are connectable to a power source (not shown). Further, an electric coil 208 may be provided around the furnace body for rotating electric arcs generated by the electrodes inside the furnace. The reduction furnace 200 further comprises a tapping zone (not shown) at its bottom. The tapping zone comprises a channel/runner through which the silicon formed in the reduction furnace 200 may flow out of the reduction furnace 200. The inside of the tapping zone, including the channel, is composed: of the inner lining 202. The inner lining 202 is thus the only material to contact the silicon formed. The reduction furnace 200 may have an upper portion which may rotate around the first electrode independently of the lower portion. An exemplary industrial setting of the reduction furnace of figure 2 is described with reference to figure 3. In the setting 300, the reduction furnace 301 , having a tapping arrangement/tapping zone 302, is arranged such that liquid silicon at the bottom of the reduction furnace 301 may pour through a channel of the tapping zone 302 into a ladle 303 placed next to the reduction furnace 301.

An operator may use tapping equipment 304 to control the tapping process. The first electrode 305 is attached to the roof, optionally in the building construction, above the reduction furnace 301 such that it may be moved between an upper and a lower position. The second electrode 306 is arranged as in figure 2. A smoke hood 307 for collecting gases formed in the process is also attached to the roof and arranged above the reduction furnace 301. The reduction furnace 301 may be split into an upper portion and a lower portion including the bottom of the furnace 301 , and the upper portion may be rotatably attached to the lower portion.

Examples 1.2:

Compaction measurement: A cylinder (diameter 50 mm, height 50 mm) of the silicon containing material is compressed with a stamp of 1 kg in a press. The compression (volume reduction) due to the weight of the stamp "forceless" relates to the pre- compression. Afterwards a Force/Stroke Diagram (force-displacement of punch ways) is measured until strength coherence. The silicon containing material possesses for this one dimensional compression a factor of 2.4 this relates to a punch way of 30 mm.

Example Electrode 1 :

A green electrode was formed (green electrode) and produced from a silicon containing material of the following composition: Silicon containing material:

90 % (w/w) silicon carbide paste; 7.5 % (w/w) silicon product (sunsil®, purity >99.9999 %, metals including iron < 5 ppma), primary particle 100 to 200 nm, 2-5 μηι secondary particle, porosity up to 97 %); 2,5 % (w/w) Novolack. The material was mixed and formed to a cylindrical shape and cured, tempered and heated according to the process parameters described below. The material was also used to form an inner lining of a reduction furnace body.

Analytical method: IPC-MS

Ga < 0.1 Sm < 0.05

Gd <0.05 Sn 3,7

Ge < 0.5 Sr 0.30

Hf < 0.05 Ta < 5

Hg < 0.01 Tb <0.05

Ho <0.05 Te < 0.01

1 < 0.1 Th <0.01

In Binder Ti 0.9

Ir < 0.01 Tl < 0.05

K 0,85 Tm < 0.05

La 11 U < 0.01

Li 0,07 V 0,24

Lu <0.05 W 1.1

Mg 1,5 Y 0.10

Yb < 0.05

Zr 0,35 Zn 26

Fraction SiC for ramming paste:

Process for curing, tempering and heating: The process steps (i) and (ii) are especially critical and a reduced heating rate was used. Step (iii) was not quite as critical, but somewhat reduced speed was selected. The silicon containing extrudable material paste, in particular mass, was protected from oxidation during this process. Argon was filled in the furnace so that the atmosphere is reducing. The furnace was closed and protected from oxygen. Therefore, the furnace was basically sealed, as is was needed . With a more or less gas tight hood and with a inert gas inlet close to the bottom.

From 900 - 1000 °C (degrees) and up to 1400 °C it was switched from argon to nitrogen to get a solid bound nitride connection (S13N4) between added Si-metal and nitrogen.

In a second production process for furnace electrode nitrogen was used during the whole heating process.

Example Electrode 2 - Silicon containing material:

90 % (w/w) silicon carbide paste, 5 % (w/w) silicon product (sunsil®, purity >99.9999 %, metals including iron < 5 ppma), primary particle 100 to 200 nm, 2-5 pm

secondary particle, porosity up to 97 %) 5 % (w/w) sucrose.

Composition of paste according to table 1 and 2 (see above).

The material was mixed and formed to a cylindrical shape and cured, tempered and heated according to the process parameters described below.

Claims

1. A component of a plant for production of silicon by carbothermic reduction,

characterized in said component comprises:

(iii) 0.1-5 % (w/w) of a reaction product of the binding agent and/or binding agent, such as a resin, e.g. a synthetic resin, and, wherein the sum of this composition is 100 % (w/w).

2. Component of a plant for production of silicon by carbothermic reduction, according to claim 1 , characterized in that said component is selected from the inner lining of a reduction furnace body, inner lining of SiC furnace, inner lining of heating or drying oven, inner lining of pelletizing, tabletting, briquetting equipment, inner lining of extruder, electrode component, electrode, tapping channel, supply channel, component of tapping equipment, ladle, crucible, inner lining of holding furnace, inner lining of directional solidification furnace, lining of devices that are in contact with quartz, silicon carbide, silicon products or formulation comprising any of quartz, silicon carbide or silicon product, inner lining of supply channels and inner lining of supply pipes.

3. Component according to claim 1 or 2, wherein the sum of all other metals (excluded silicon) in the silicon carbide product or the silicon product, respectively, is less than 100 ppm (w/w), wherein particularly said other metals are selected from Ag, Al, As, Au, Ba, Be, Bi, Ca, Cd, Ce, Co, Cr, Cs, Cu, Dy, Er, Eu, Fe, Ga, Gd, Ge, Hf, Hg, Ho, In, Ir, K, La, Li, Lu, Mg, Mn, Mo, Na, Nb, No, Os, Pb, Pd, Pr, Pt, Rb, Re, Rh, Ru, Sb, Sc, Se, Sm, Sn, Ta, Tb, Te, Th, Ti, Tl, Tm, U, W, Y, Yb, Zn, Zr or wherein the sum of all of said other metals in the silicon carbide product and the silicon product is less 100 ppm (w/w), particularly less than 80 ppm (w/w). Component according to any of claims 1 to 3, wherein said other metals in the silicon carbide product and the silicon product selected from the following is equal or less than:

a. aluminium (Al) 30 ppm (w/w) or from 25 to 0.0001 ppt (w/w) , particularly from 22 ppm (w/w) to 0.0001 ppt (w/w), and

b. boron (B) 1.0 ppm (w/w) to 0.0001 ppt (w/w) , particularly from 0.5 ppm (w/w) to 0.0001 ppt (w/w), preferred from 0.

4 ppm (w/w) to 0.0001 ppt (w/w) or particularly preferred 0.35 ppm (w/w) to 10 ppb (w/w); and

f. phosphorus (P) 3 ppm (w/w) to 0.0001 ppt (w/w), particularly from 2 ppm (w/w) to 0.0001 ppt (w/w) .preferred from 1 ppm (w/w) to 0.0001 ppt (w/w), and g. titan (Ti) 2 ppm (w/w), particularly from 1.7 ppm (w/w) to 0.0001 ppt, preferred from 1.5 to 0.0001 ppt (w/w); and

h. zinc (Zn) 30 ppm (w/w), particularly from 25 ppm (w/w) to 0.0001 ppt (w/w) preferred from 10 ppm to 0.0001 ppt (w/w).

Component according to any of claims 1 to 4, characterized in that the componnt is a green body and comprises a composition selected from

(ii) 5-20 % (w/w) of a silicon product containing 1.

5 ppm (w/w) or less boron and 3.0 ppm (w/w) or less phosphorus; wherein the silicon product is silicon, and

(iii) 0.1-5 % (w/w) of a binding agent, such as a resin, a carbohydrate, e.g. a synthetic resin, a phenol- formaldehyde resins and/or a saccharide; and, wherein the sum of this composition is 100 % (w/w).

6. Component according to any of claims 1 to 4, characterized in said component is a tempered body comprises a composition selected from

(ii) 5-20 % (w/w) of a silicon product containing 1.5 ppm (w/w) or less boron and 3.0 ppm (w/w) or less phosphorus; wherein the silicon product comprises silicon nitride and optionally silicon, and

(iii) 0 to 5 (w/w) (w/w) of a reaction product of the binding agent, such as a resin, a carbohydrate, e.g. a synthetic resin and/or saccacharide, and, wherein the sum of this composition is 100 % (w/w).

7. A reduction furnace body for production of liquid silicon by carbothermic

reduction, said body comprising an inner lining, characterized in that said inner lining comprises:

(ii) 0.1-5 % (w/w) of a reaction product of the binding agent and/or binding agent, such as a resin, e.g. a synthetic resin; and, in particular wherein the sum of this composition is 100 % (w/w).

8. A reduction furnace body according to claim 7, wherein the silicon carbide product and the silicon product each contain 300 ppm (w/w) Al or less.

9. A reduction furnace body according to claim 7 or 8, wherein the sum of all metals in the silicon carbide product and the silicon product, respectively, is less than 500 ppm.

10. A reduction furnace body according to any one of claims 7 to 9 comprising a second lining composed of a material which is different from the inner lining material, said second lining being arranged to contact the outer surface of the inner lining.

11. A reduction furnace body according to claim 10, wherein the material of the second lining is silica stone or silica ramming paste.

12. A reduction furnace body according to claim 10 or 11 , wherein a layer of graphite or carbon ramming paste is arranged in between the inner and the second lining, in the bottom of the body.

13. A reduction furnace body according to any one of the preceding claims, wherein the binding agent is a resin selected from synthetic phenol- formaldehyde resins.

14. A reduction furnace body according to any one of the preceding claims, wherein a closable channel between the interior and the exterior of the body for tapping of liquid silicon is provided at the bottom of the body and wherein the inner walls of the channel are composed of said inner lining.

15. A reduction furnace body according to any one of the preceding claims which is split into an upper and a lower portion, wherein the upper portion is rotatably attached to the lower portion.

16. Electrode component or electrode, characterized in that at least a component of said electrode comprises a silicon containing material with a composition selected from

(i) 80 to 95 % (w/w) of a silicon carbide product containing 1.5 ppm (w/w) or less boron and 3.0 ppm (w/w) or less phosphorus;

(ii) 5 to 20 % (w/w) of a silicon product containing 1.5 ppm (w/w) or less boron and 3.0 ppm (w/w) or less phosphorus; and

(iii) 0 to 5 (w/w) of a reaction product of a binding or a binding agent, such as a resin, e.g. a synthetic resin; and, wherein the sum of this composition is 100 % (w/w).

17. Electrode according to claim 16, characterized in that said electrode is a green electrode, furnace electrode, particularly a furnace electrode for a reduction furnace, preferred a furnace electrode for a reduction furnace for the production of liquid silicon, more preferred a furnace electrode for a reduction furnace for the production of liquid silicon by carbothermic reduction, a furnace bottom electrode, or at least a component of said green electrode.

18. Electrode according to any of claims 1 to 7, wherein said silicon containing

material contains equal or less than 1.5 ppm (w/w) silver, particularly less than 1.0 ppm (w/w) boron (B), preferred less than 0.5 ppm (w/w), more preferred less than 0.2 ppm (w/w) and/or less than 2.0 ppm (w/w) phosphorus (P), particularly equal or less than 1.0 ppm (w/w) phosphorus and optional equal or less than 5 ppm (w/w) sulphur (S), particularly less than 4.5 ppm (w/w) sulphur.

19. Ladle, crucible according to any claims 1 to 6, wherein said ladle and/or crucible comprises at least an inner lining comprising:

(iii) 0.1-5 % (w/w) of a reaction product of the binding agent and/or binding agent, such as a resin, e.g. a synthetic resin; and, wherein the sum of this composition is 100 % (w/w), in particular the ladle further comprises an arrangement of heating means, preferred heating means are inductive heating means.

20. Process for the production of a component of a plant for production of silicon by carbothermic reduction, wherein a green component comprising

(iii) 0.1-5 % (w/w) of the binding agent and/or binding agent, such as a resin, e.g. a synthetic resin; and, wherein the sum of this composition is 100 % (w/w), - is formed and optionally cured.

. Process according to claim 20, with the steps of

(i) curing of the formed green component, and optional (ii) tempering of the green component and optional

22. Process according to claim 20 or 21 , wherein said green component selected from the inner lining of a reduction furnace body, inner lining of SiC furnace, inner lining of heating or drying oven, inner lining of pelletizing, tabletting, briquetting equipment, inner lining of extruder, electrode component, electrode, tapping channel, supply channel, component of tapping equipment, ladle, crucible, inner lining of holding furnace, inner lining of directional solidification furnace, lining of devices that are in contact with quartz, silicon carbide, silicon products or formulation comprising any of quartz, silicon carbide or silicon product, inner lining of supply channels and inner lining of supply pipes.

23. Process according to any of claims 20 to 22, wherein the curing, tempering

and/or heating is performed in an atmosphere of an inertgas as argon.

24. Process according to any of claims 20 to 23, wherein the curing, tempering

and/or heating is performed in an atmosphere containing nitrogen, particular the heating step is performed in an nitrogen containing atmosphere, preferred in a nitrogen atmosphere, e.g. from 10 to 100 Vol.-% N₂ ad 100 Vol.-% of inertgas particular argon.

25. Process according to any of claims 20 to 24, wherein the green component is formed of a composition comprising

(i) 85-95 % (w/w) of a silicon carbide product containing 1.5 ppm (w/w) or less boron and 3.0 ppm (w/w) or less phosphorus;

(ii) 5-15 % (w/w) of a silicon product containing 1.5 ppm (w/w) or less boron and 3.0 ppm (w/w) or less phosphorus; and

26. Process according to any of claims 20 to 25, wherein the temperature (i) during the curing step is from 0 to 300 °C, in particular form 50 to 300 °C, preferred from 100 to 250 °C, particularly preferred from 150 to 250 °C, each plus/minus 25 °C, and optional

(ii) during the tempering step from 200 to 500 °C, in particular from 200 to 450 °C, preferred from 250 to 400 °C each plus/minus 25 °C, and optional

(iii) during the heating step from 350 to 1400 °C, in particular in several steps from 400 to 600 °C or from 600 °C to 1400 °C, or from 600 to 900 °C and from 900 to 1400 °C.

27. Process according to any of claims 20 to 26, wherein the temperature setting is +5 °C/hour, in particular + 10 °C/hour or + 15 °C/hour in step (i), (ii) and/or (iii).

28. Process according to any of claims 20 to 27, wherein the holding time in each step of (i), (ii) and/or (iii) independently is from 10 to 250 hours, in particular the holding time per step is from 25 to 150 hours, preferred from 50 to 100 hours, in particular in step (i) and (ii).

29. Component of a plant for production of liquid silicon by carbothermic reduction obtained by a process of any of claims 20 to 28.

30. Component of claim 29, characterised in that it is selected from the inner lining of a reduction furnace body, inner lining of SiC furnace, inner lining of heating or drying oven, inner lining of pelletizing, tabletting, briquetting equipment, inner lining of extruder, electrode component, electrode, tapping channel, supply channel, component of tapping equipment, ladle, crucible, inner lining of holding furnace, inner lining of directional solidification furnace, lining of devices that are in contact with quartz, silicon carbide, silicon products or formulation comprising any of quartz, silicon carbide or silicon product, inner lining of supply channels and inner lining of supply pipes, from electrode strand, electrode body, nipple, blocking pin, clamping jaws, mounting or green body of any of them or electrode, such as bottom electrode or electrode assembly comprising at least two of the aforementioned electrode components.

31. Use of a component of a plant for production of liquid silicon by carbothermic reduction for production of a silicon product containing 1.5 ppm (w/w) or less boron and 3.0 ppm (w/w) or less phosphorus by carbothermic reduction, comprising the steps of:

(i) 80-95 % (w/w) of a silicon carbide product having at least the same purity as the silicon carbide product added in step a),

(ii) 5-20 % (w/w) of a silicon product, in particular comprising silicon nitride and optionally silicon, having at least the purity of the silicon product produced by step a) and b); and

(iii) 0-5 % (w/w) of a reaction product of a binding agent, such as a resin or carbohydrate.

32. Method for production of a silicon product containing 1.5 ppm (w/w) or less boron and 3.0 ppm (w/w) or less phosphorus by carbothermic reduction, comprising the steps of: - adding a quartz product containing 0.50 ppm (w/w) or less boron and 1.0 ppm (w/w) or less phosphorus and a silicon carbide product containing 1.5 ppm (w/w) or less boron and 3.0 ppm (w/w) or less phosphorus; to a reduction furnace having an inner lining; and

b) heating the mixture such that a melt of the silicon product is formed, wherein said lining comprises: 80-95 % (w/w) of a silicon carbide product having at least the same purity as the silicon carbide product added in step a) 5-20 % (w/w) of a silicon product having at least the purity of the silicon product produced by the method; and 0.1-5 % (w/w) of a binding agent or a reaction product of the binding agent, such as a resin.